The mechanisms by which the extracellular matrix influences a cell's decision to proliferate or differentiate are poorly understood. During the last decade, many extracellular matrix-binding proteins were described and they are now known to be members of a large family of integrin receptor II IL it is becoming appreciated that integrins not only bind to matrix components but also interact with.the cytoskeleton and initiate signal transduction pathways that may be critical for cellular proliferation and differentiation. The Schwann cell (SC), the myelin forming cell of the peripheral nervous system, is an example of a cell that is absolutely dependent on the deposition of extracellular matrix in order to differentiate in response to axonal signals expressed by neurons. Although this observation was made nearly 15 years ago, we are only now ready to delineate the molecular mechanisms underlying this interdependence of extracellular matrix deposition and SC differentiation. This proposal provides experimental strategies that will allow ascertainment of the following: 1) whether the alpha6beta1 and alpha 6 Beta4 integrins, known to be expressed by SCs, play a role in SC differentiation into myelin-forming cells, 2) whether Beta1 or Beta4 integrin activation, or the actin-based cytoskeleton, are associated with expression of myelin-specific proteins or their mRNAs, 3) whether Beta1 or Beta4 integrins interact with actin, actin binding proteins, and signaling molecules such as the newly-described focal adhesion kinase, to form a stable transmembrane complex that forms upon SC binding to the extracellular matrix, and 4) whether tyrosine phosphorylation events are associated with and necessary for integrin-mediated signal transduction. These studies will be conducted in co-cultures of SCs with sensory neurons that allow 1) precise control of SC proliferation and differentiation, 2) perturbation of the expression of single molecules hypothesized to play critical roles in SC differentiation, and 3) that are amenable to analysis using cellular, molecular, biochemical, and morphological techniques. The information provided by these studies is crucial to our understanding of the mechanisms controlling normal cell development and will be directly applicable to efforts to control tumorigenesis in diseases such as neurofibromatosis, as well as to develop therapeutic strategies to alleviate demyelination caused by injury to the nervous system or by attack on the nervous system by immune mechanisms.